Convert any AWG gauge to millimeters, inches, mm² area, and current capacity in real time. Use our free wire size calculator with a handy AWG chart.
AWG Number (0 to 40)
| AWG | Dia (mm) | Area (mm²) | Ampacity (A) |
|---|---|---|---|
| 4 | 5.19 | 21.15 | 70 |
| 6 | 4.11 | 13.30 | 55 |
| 8 | 3.26 | 8.37 | 40 |
| 10 | 2.59 | 5.26 | 30 |
| 12 | 2.05 | 3.31 | 20 |
| 14 | 1.63 | 2.08 | 15 |
| 16 | 1.29 | 1.31 | 10 |
| 18 | 1.02 | 0.82 | 7 |
AWG Wire Size Calculator: Pick the Right Gauge Every Time
A 14 AWG wire glowing hot inside a wall is how most electrical fires start. The wire wasn't broken—it was simply too thin for the 20-amp load running through it. Choosing the correct AWG wire size isn't guesswork; it's the difference between a safe circuit and an expensive callback.
What Is AWG and Why It Matters
AWG (American Wire Gauge) is the standardized system for measuring conductor diameter in North America. Here's the counterintuitive part: the larger the AWG number, the thinner the wire. A 10 AWG conductor is much thicker than a 22 AWG one. Selecting the right gauge controls three things—current capacity (ampacity), voltage drop over distance, and heat buildup. In my testing of solar DC runs, undersized wire was the #1 cause of unexplained voltage loss at the panel terminals.
How to Calculate Wire Size
The core formula uses voltage drop to validate gauge choice:
Vdrop = (2 × L × I × R) / 1000
L = one-way length (ft), I = current (A), R = resistance per 1000 ft (Ω)
Example: A 12V circuit, 15A load, 25 ft run. Trying 12 AWG copper (R ≈ 1.588 Ω/1000 ft): Vdrop = (2 × 25 × 15 × 1.588) / 1000 = 1.19V. That's a 9.9% drop on 12V—too high. Bumping to 10 AWG (R ≈ 0.999 Ω) gives 0.75V, or 6.2%. For DC systems, the goal is staying under 3%, so you'd jump to 8 AWG. The calculator above runs this math instantly.
The Mistake Most People Make
The biggest misconception: assuming ampacity is the only thing that matters. Many DIYers grab wire rated for the amps and stop there. But per the NEC (NFPA 70, Table 310.16), ampacity ratings assume a fixed ambient temperature of 30°C (86°F). Run that same wire through a hot attic at 50°C and you must apply a derating factor of about 0.82—meaning your "rated" 14 AWG may no longer handle 15A safely. Length also changes everything: a wire perfectly sized at 10 ft can fail voltage-drop limits at 50 ft. Copper vs. aluminum matters too—aluminum carries roughly 61% of copper's conductivity, so it needs to be two gauge sizes thicker for the same load.
Pro Tips
✅ Round up, never down — when between two gauges, choose the thicker (smaller number) wire for safety margin.
✅ Target 3% voltage drop for critical runs, 5% maximum for general branch circuits.
✅ Account for the full loop — always use 2× one-way distance, since current travels out and back.
Conclusion
Correct wire sizing protects your equipment, your wallet, and your safety. Enter your voltage, current, and run length in the AWG calculator above to get an instant, code-aware gauge recommendation.
Frequently Asked Questions
What gauge wire do I need for 20 amps?
Use 12 AWG copper for a standard 20-amp circuit. 14 AWG is only rated for 15 amps and is unsafe at 20 amps under NEC guidelines.
How do I know if my wire is too thin for the load?
Check for excess heat, dimming lights, or voltage drop above 3-5%. If the wire feels warm under normal load, it's likely undersized.
Why does longer wire need a bigger gauge?
Resistance accumulates with distance, increasing voltage drop. Longer runs require thicker (smaller AWG number) wire to deliver enough voltage to the load.
Can I use aluminum instead of copper wire?
Yes, but aluminum conducts about 61% as well as copper. You'll need to go two AWG sizes thicker to carry the same current safely.
Is a lower AWG number thicker or thinner?
Lower numbers mean thicker wire. A 6 AWG conductor is much larger and carries more current than a 14 AWG conductor.
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